This invention relates to a modified dielectric oxide film on the surface of an aluminum electrolytic capacitor anode. More particularly, it relates to a dielectric oxide film stabilized by the incorporation of a minor amount of a carbonaceous material. The invention permits the utilization of aluminum foils which have a very fine etch structure, as anodes in capacitors.
Anodized aluminum anodes enjoy widespread use in electrolytic capacitors. One problem associated with them is the degradation of the anhydrous aluminum oxide dielectric by moisture resulting in hydration of the dielectric which produces a conductive layer and an increase in the equivalent series resistance (ESR) of the capacitor. Since it is quite difficult to maintain perfectly anhydrous conditions, one solution to this problem has been to produce a hydrated surface to protect the underlying oxide layer to prevent such degradation, particularly in use in the final capacitor.
With the production of a finer etch structure on aluminum foils, deliberate hydration must be limited. The largest part of the available surface area in these finely etched foils is in the form of long, narrow channels or tubes running beneath the foil surface; excess hydrous oxide plugs these channels with the result that portions become inaccessible to the working electrolyte of the capacitor.
It has been possible to remove this unwanted hydrous oxide from the surface of the films by various treatments during manufacture. For example, a process patented by Alwitt (U.S. Pat. No. 3,733,291 issued May 15, 1973) involves stripping away any hydrated oxide that remains after anodization, and our copending application (Ser. No. 791,656 filed April 28, 1977) involves inhibiting hydration during and subsequent to depolarization while even dissolving some of the unwanted hydrous oxide present. The first process primarily affects the surface of the foil, while the second process does work throughout the pore structure. However, neither of these appear to modify the film by incorporation of material into it.
The presence of heavy hydrous oxide layers is troublesome in the formation of high voltage foils, as at higher voltage the barrier oxide layer formed is thicker and tends to clog the etch tunnels. In order to keep the thickness of total layer (oxide plus hydrate) as low as possible, it is desirable to minimize hydrate formation.
While the additives of the present invention have been used as solutes in formation electrolytes, the resulting dielectric oxide films have not been as satisfactory as those produced in conventional boric acid/borate formation electrolytes. Some working electrolytes also contain these additives; again, the action is a surface one without modifying the oxide layer.
It has been recognized that material from a formation electrolyte can be incorporated into the dielectric oxide layer during anodization. This has been described in the technical literature including a paper by J. J. Randall, Jr., W. J. Bernard, and R. R. Wilkinson published in Electrochimica Acta 10: 183-201(1965), and another paper by J. J. Randall, Jr., and W. J. Bernard published in Electrochimica Acta 20: 653-61(1975).